1. Field of the Invention
The present invention relates to a solid-state laser oscillator using solid-state laser rods. More particularly, this invention is concerned with a solid-state laser oscillator having a high average-power transverse single-mode resonator which uses Nd:YAG rods to be excited by a high average-power laser diode.
2. Description of the Related Art
An active medium in a transverse single-mode oscillator is excited by an exciting light source such as a laser diode or a flash lamp. The excited active medium or any optical component is used to achieve transverse single-mode laser oscillation. This results in a transverse single-mode output. There are many approaches to exciting of an active medium. One of the approaches is end exciting.
End exciting is an exciting method for exciting a laser medium which is placed substantially along the optical axis of a resonator. The exciting is performed from the end of the laser medium, wherein a laser diode is mainly used for exciting. Exciting light output from the laser diode that is placed substantially along the optical axis of the resonator is incident substantially perpendicularly on an end of an active medium coated with an antireflection coating that is non-reflective to light having the same wavelength as the wavelength of the exciting light. The light is then absorbed into the active medium. The active medium is thus excited. The resonator consists of the excited active medium, total reflection mirrors, a partial reflection mirror, and an arbitrary optical component. The total reflection mirrors are located ahead of and behind the active medium, placed substantially along the optical axis of the resonator, and have a property of totally reflecting light having the same wavelength as the wavelength of laser light. The partial reflection mirror reflects part of the light having the same wavelength as the wavelength of the laser light. The excited active medium has electrons of high energy states made a transition to a lower energy state that is a stable state. At this time, photons are emitted. The total reflection mirrors and partial reflection mirror included in the resonator cause the photons to orbit. This stimulated emission performed by the active medium causes laser light of a specified wavelength to be amplified. Part of the laser light is emitted from the partial reflection mirror.
According to the end exciting method, the directivity of the laser diode is utilized in order to excite the solid-state laser medium so that transverse single-mode resonant laser beam alone will be propagated. Consequently, transverse single-mode laser oscillation is achieved highly efficiently. However, an output being generated from a single stripe in the laser diode is limited because the end of the laser diode is destroyed. For providing a high-power output, the number of laser diodes must be increased. This leads to deterioration in the directivity of the laser diode. Consequently, it becomes hard to excite the solid-state laser medium so that the transverse single-mode resonant laser beam alone will be propagated. Furthermore, since exciting light is converged on a microscopic area on the end of the solid-state laser medium, the power density of the exciting light is generally high. When the average power of exciting light is raised, the solid-state laser medium may be thermally destroyed with the exciting light. Existing transverse single-mode oscillators adopting the end exciting method have therefore been limited to applied fields in which low average-power laser light is needed.
High average-power solid-state lasers therefore adopt side exciting. Side exciting is an exciting method for exciting an active medium in a direction perpendicular to the optical axis of a resonator using an exciting source such as a laser diode or a flash lamp.
A transverse mode dependent on a laser medium is determined with resonance conditions. A transverse single mode is considered as a sort of pattern exhibited by laser light whose beam radius of the cross section of the laser light with the ray axis thereof as a center is the smallest. Low-order and high-order transverse modes are considered as sorts of patterns exhibited by laser light having larger beam radii. Now, assume that the laser medium itself is thought to serve as a mode selection aperture. If the size of an excited laser medium is equivalent to the beam radius of the laser light exhibiting the transverse single mode, the high-order transverse modes are not selected but the transverse single mode is selected. In contrast, when the size of the excited laser medium is larger than the beam radius of the transverse single-mode laser light, a high-order transverse mode is selected. At this time, laser oscillation is achieved to generate laser light exhibiting a multi-mode that is a combination of a plurality of waveguide modes including the transverse single mode, low-order modes, and high-order modes. A laser output is therefore multi-mode laser light. Multi-mode laser light is poorer in directivity than the transverse single-mode laser light. As the multi-mode laser light is propagated, it spreads widely. The multi-mode laser light is characterized in that when an attempt is made to converge the multi-mode laser light on a lens or the like, the cross section of the multi-mode laser light is not narrowed. Compared with a transverse single-mode laser, therefore, a multi-mode laser is of little worth for the purposes of configuring laser equipment that utilizes propagation of laser light or of performing machining with converged laser light.
For producing transverse single-mode light highly efficiently, the beam radius of transverse single-mode light propagated in a resonator must be equivalent to the size of a laser medium.
For improving the average power of transverse single-mode laser light, a laser medium must be excited with high-power exciting light. When the laser medium is excited with high average-power light, heat is generated in the laser medium due to the exciting light. Generation of heat optically distorts the laser medium. The thermal distortion leads to a loss of laser light orbiting within a resonator while being amplified. A gain of laser light to be produced by the resonator increases proportionally to the power of exciting light. However, as long as transverse single-mode laser oscillation is concerned, when the magnitude of thermal distortion is small, a loss stemming from thermal distortion increases in proportion to the square of the magnitude of thermal distortion. When the laser medium is excited with high-power exciting light, the loss increases more greatly than an increase in the gain of laser light produced by the resonator. The maximum power of laser light is therefore limited. For efficiently performing laser oscillation so as to generate high average-power transverse single-mode light, it is necessary to minimize the thermal distortion of the laser medium.
In general, a rod-shaped solid-state laser medium is referred to as a solid-state laser rod. When the solid-state laser rod is excited, heat is generated. The solid-state laser rod is therefore cooled with a coolant placed by the side thereof. Heat is distributed over the cross section of the solid-state laser rod. This results in a difference in temperature causing distribution of refractive indices. In particular, when the solid-state laser rod is realized with an isotropic medium made of an isotropic crystal of yttrium aluminum garnet (Y3Al5O12) with an atom of neodymium (Nd) appended thereto (hereinafter Nd.YAG), the solid-state laser rod acts as a convex lens (which may be referred to as a heat lens) relative to laser light. As the power of exciting light is raised, the heat lens effect is intensified (the focal length of the solid-state laser rod gets shorter).
When a laser medium brings about any kind of birefringence, a heat lens effect exerted from the laser medium also provides a doublet lens effect. For designing a resonator capable of generating transverse single-mode light, the stability criteria of the resonator must be determined in consideration of the doublet lens effect. Nd:YAG is an isotropic crystal. However, birefringence occurs due to a photoelastic effect that relates to a stress stemming from distribution of temperatures. In particular, when a rod-shaped laser medium is employed, birefringence occurs due to radial polarization or peripheral polarization.
For resolving the foregoing drawback, two equally excited rod-shaped laser media, that is, two equally excited solid-state laser rods are employed, and a 90xc2x0optical rotator is interposed between the rods. The focal lengths offered by the rods and affected by the heat lens effect are thus averaged. This technique has proved effective.
A resonator that acts to efficiently generate high-power transverse single-mode light must be designed in such a manner that when a Nd:YAG rod serving as a mode selection aperture is excited from the side surface thereof, the beam radius of light coming out of the rod becomes optimal with application of maximum-power exciting light. On the other hand, as the power of exciting light is raised, the heat lens effect is intensified. At this time, a range of values in the stability domain within which the stability criteria of the resonator are set gets smaller in inverse proportion to the square of a beam radius. It becomes hard to retain the stability criteria in the small range of values despite application of high-power exciting light. For achieving transverse single-mode laser oscillation, the beam radius must be as large as the diameter of the rod serving as the mode selection aperture. Therefore, it is hard to achieve laser oscillation with application of high-power exciting light because the application of high-power exciting light intensifies the heat lens effect. Transverse single-mode laser oscillation is therefore limited to laser oscillation to be performed for generating low-power laser light with application of low-power exciting light because the heat lens effect remains suppressed with application of low-power exciting light.
As a solving system, a heat lens compensating device, such as a lens or a curvature mirror, is placed in a resonator. This is intended to shift an action point in the stability domain, which indicates the stability criteria of the resonator, to a range of values in the stability domain indicating the stability criteria thereof for acting on application of high-power exciting light. In this case, since the intense heat lens effect stemming from application of high-power exciting light is compensated, laser oscillation cannot be achieved with application of low-power exciting light. Moreover, when the heat lens effect is intensified, the range of values in the stability domain within which the stability criteria of the resonator can be set gets smaller in inverse proportion of the square of the beam radius of laser light. Therefore, when high-power exciting light is applied, the stability criteria of the resonator are confined to a smaller range of values in the stability domain. A difference from threshold power of exciting light to power resulting in a maximum laser output diminishes. A slight variation in the power of exciting light may presumably cause a large variation in the laser output. From this viewpoint, laser oscillation for generating high average-power transverse single-mode light has thought to be hard to realize because the intense heat lens effect caused by application of high-power exciting light must be compensated.
FIG. 11 shows the configuration of a known transverse single-mode resonator described in, for example, xe2x80x9cSolid-state Laser Engineeringxe2x80x9d written by Walter Koechner (4th Edition, Springer Series in Optical Science, Vol. 1, P.215). Referring to FIG. 11, there are shown a transverse single-mode resonator 101, a first exciting source 103-1, a second exciting source 103-2, a first solid-state laser rod 104-1, a second solid-state laser rod 104-2, a 90xc2x0 optical rotator 105, a reflecting device 107, a partial reflection device 108, a Brewster plate 109, transverse single-mode light 130, and transverse single-mode output light 131.
Referring to FIG. 11, the first solid-state laser rod 104-1 and second solid-state laser rod 104-2 are placed mutually coaxially in parallel with each other, The first solid-state laser rod 104-1 and second solid-state laser rod 104-2 absorb exciting light emitted from the first exciting source 103-1 and second exciting source 103-2 respectively located near the associated solid-state laser rods. The first solid-state laser rod 104-1 and second solid-state laser rod 104-2 are thus excited. The 90xc2x0 optical rotator 105 is interposed between the first solid-state laser rod 104-1 and second solid-state laser rod 104-2 and placed coaxially with the solid-state laser rods. The first solid-state laser rod 104-1 and second solid-state laser rod 104-2 are realized with Nd:YAG lasers. When the solid-state laser rods are excited from the exciting sources, they exert a heat lens effect or a heat doublet lens effect. The inclusion of the two solid-state laser rods and 90xc2x0 optical rotator enables compensation of the heat doublet lens effect.
The reflecting device 107 and partial reflection device 108 are placed coaxially with the solid-state laser rods and arranged perpendicularly outside the two solid-state laser rods. The reflecting device 107 has a convex reflecting surface, while the partial reflection device 108 has a concave partial reflection surface. The reflecting device 107 and partial reflection device 108 compensate the heat lens effects exerted from the two solid-state laser rods. Part of transverse single-mode light orbiting between the reflecting device 107 and partial reflection device 108 and having been amplified by the solid-state laser rods is transmitted by the partial reflection device 108. Consequently, the transverse single-mode output light 131 is provided as a laser output of the transverse single-mode resonator 101. The Brewster plate 109 is placed on the optical axis of the transverse single-mode resonator while tilted by a Brewster angle with respect to the optical axis of the transverse single-mode resonator. The transverse single-mode light has a linearly polarized light component thereof selected. The transverse single-mode output light 131 is therefore linearly polarized light.
In the related art shown in FIG. 11, a distance between the first solid-state laser rod 104-1 and reflecting device 107, a distance between the second solid-state laser rod 104-2 and partial reflection device 108, the curvature of the convex surface of the reflecting device 107, and the curvature of the concave surface of the partial reflection device 108 are varied arbitrarily. Thus, the beam radius of single-mode light coming out of the solid-state laser rods is made nearly equal to the radius of the solid-state laser rods. The transverse single-mode resonator is thus configured to generate transverse single-mode light,
However, in the foregoing configuration, the heat doublet lens effect is not fully compensated for some reasons. One of the reasons is that the configuration of the resonator is asymmetric. The heat lens effect is exerted in a radial direction and a peripheral direction relative to a cross section of each solid-state laser rod. Since the radial and peripheral heat lens effects are exerted, the stability criteria of the resonator are set to fall within different ranges of values in the stability domain. Assume that the power of exciting light to be applied to the solid-state laser rods is low and the heat lens effects exerted by the solid-state laser rods are feeble. In this case, the beam radius of transverse single-mode light coming out of the solid-state laser rods may be made as large as the radius of the solid-state laser rods in order to provide a transverse-single mode output. As mentioned above, the stability criteria of the resonator are set to the different ranges of values in the stability domain in consideration of the radial and peripheral heat lens effects. The different ranges of values in the stability domain are largely overlapping. After the radial and peripheral heat lens effects are compensated, transverse single-mode laser oscillation can be achieved with the stability criteria set to fall within a common range between the overlapping range of values. In contrast, assume that the power of exciting light to be applied to the solid-state laser rods is high and the heat lens effects exerted by the solid-state laser rods are intense. For providing a transverse single-mode output, the beam radius of transverse single-mode light coming out of the solid-state laser rods is made as large as the radius of the solid-state laser rods. AS mentioned above, since the heat lens effects are intense, the stability criteria of the resonator are confined to a small range of values in the stability domain. Moreover, the stability criteria of the resonator must be set to fall within different ranges of values in the stability domain in consideration of the radial and peripheral heat lens effects. Besides, since the different ranges of values overlap a little, a common range between the overlapping ranges of values is very small. Consequently, the stability criteria of the transverse single-mode resonator are confined to the very small range of values. It is therefore hard to achieve laser oscillation.
The stability criteria of the transverse single-mode resonator are set to the small range of values in the stability domain so that the resonator will act optimally on application of high-power exciting light. Therefore, a difference between power of exciting light permitting laser oscillation and power of exciting light needed to provide a maximum laser output is very small. A laser output is therefore quite sensitive to a small variation in the power of exciting light and less stable.
As mentioned above, assuming that the related transverse single-mode resonator shown in FIG. 11 is used to construct a solid-state laser oscillator, once the intense heat lens effects exerted by the solid-state laser rods responsively to application of high exciting power are compensated, the stability criteria of the resonator are confined to a narrow range of values in the stability domain. Consequently, laser oscillation is hard to achieve. Even when laser oscillation is attempted, a laser output is sensitive to a slight variation in the power of exciting light and little stable.
Accordingly, an object of the present invention is to provide a solid-state laser oscillator capable of generating high average-power transverse single-mode output light on a highly stable basis with the stability criteria of a resonator set to fall within a wide range of values in the stability domain. Herein, a transverse single-mode resonator consists of two solid-state laser rods, two solid-state laser rod modules, a 90xc2x0 optical rotator, two head lens compensating device, a reflecting device, a partial reflection device, an exciting source driving power supply, a supply current line, a supply current control device, a control current line, a parameter input device, and a setting signal line.
With the above object in view, the solid-state laser oscillator of the present invention comprises: any number of solid-state laser rods disposed coaxially in parallel with each other, radiating light when excited, and amplifying the light through stimulated emission; any number of solid-state laser rod exciting device for exciting the any number of solid-state laser rods; any number of 90xc2x0 optical rotators disposed coaxially with the solid-state laser rods, arranged between the any number of solid-state laser rods, and rotating a component of the light generated in the axial direction; any number of heat lens compensating device disposed coaxially with the solid-state laser rods and located at any positions; a reflecting device and a partial reflection device, disposed coaxially with the solid-state laser rods and arranged outside all of the solid-state laser rods and heat lens compensating device, for propagating the axially generated component of the light; and an exciting source driving device for driving the exciting sources included in the solid-state laser rod exciting device, wherein a component of the light component, which is propagated between the reflecting device and partial reflection device, is transmitted by the partial reflection device and output as laser light.
The exciting source driving device may include an exciting source driving power supply for feeding a driving current used to drive the exciting sources.
The exciting source driving device may include: a parameter input unit for use in entering values of parameters, that is, driving conditions for the exciting source driving power supply; and a supply current control unit for controlling the exciting source driving power supply according to the driving conditions.